1. Technical Field
The present invention relates to an improvement of an electric power steering apparatus (hereinafter, refer to as an EPS) structured to apply a steering assist force generated by a motor to a steering system of a motor vehicle or a vehicle.
2. Prior Art
In general, in this kind of EPS, if the EPS is actuated in a state in which a battery is deteriorated in a vehicle side or a used load is increased, there is a case where a power supply voltage is decreased. In a state in which the power supply voltage is decreased, a gate driving voltage of an FET used in a motor driving circuit is decreased. In this case, as shown in FIG. 7, if a voltage (VGS) between gate and source in the FET is decreased, an on resistance (RDS(ON)) between drain and source is exponentially increased. In this connection, the following relation exists between a maximum driving current Imax and an allowable power value P. In other words, the expression P=RDS(ON)Imax2 is established. In this case, reference symbol P denotes an allowable power value of the FET, reference symbol RDS(ON) denotes an on resistance between source and drain in the FET, and reference symbol Imax denotes a motor maximum current allowing to circulate through the FET. As a result, if the value RDS(ON) becomes large at a time of controlling a drive of the motor, a power loss becomes large. Accordingly, if the power supply voltage is decreased, there is a risk that the FET is broken or burned out by a heat generation caused by the power loss of the FET.
Further, if the power supply voltage is significantly decreased, a supply voltage to a torque sensor is decreased. Accordingly, a sensor characteristics (a solid line) at a time of a normal voltage is decreased as shown by a sensor characteristics (a broken line) at a time of a voltage reduction, as shown in FIG. 8. For example, in the case where a sensor output at a time of a neutral steering is set to 2.5 V, a neutral position of a steering wheel is shifted at a value s1 in the drawing, and a current characteristics of the motor is also shifted at a value s2 in the drawing from the neutral position of the steering wheel, as shown in FIG. 9. Accordingly, a left-right difference is generated in a steering wheel steering force such that the steering force is light in a leftward steering and is heavy in a rightward steering, and in the case where the left-right difference comes off badly, a steering wheel shift or the like is generated, so that there is a problem that a steering feeling is deteriorated. As a result, in the conventional EPS, if the power supply voltage is decreased to a certain voltage value or less, an output of the EPS is forcibly turned off so as to stop the assist.
For example, in Japanese Patent No. 3328595, there is disclosed a structure which is provided with a power supply voltage detecting means and a fadeout control means, detects a power supply voltage signal of an in-vehicle power supply so as to compare with a reference value, reduces a target current signal in accordance with a time so as to reduce a current of a motor in accordance with the time in the case where the power supply voltage signal is equal to or less than a first reference value continuously for a fixed time, thereby gradually reducing an assist steering force, and set the target current signal to 0 so as to stop the assist in the case where the power supply voltage signal becomes equal to or less than a second reference value smaller than the first reference value.
Further, for example, in Japanese Patent Application Laid-open Publication No. 6-15331, there is disclosed a structure which is provided with a voltage detecting means, a current limit value setting means and a current limiting means, detects a voltage of any of feed lines of a power supply, limits a motor current in such a manner that the detected value does not exceed a current limit value, sets a current limit value which becomes smaller in accordance with a reduction of a power supply voltage, and can obtain an assist of a normal steering force in the case of such a light steering that the motor current does not reach the current limit value.
However, in the conventional structure mentioned above, in the case where the detected value of the power supply signal becomes smaller than the reference value (the current limit value), the structure immediately stops the assist, and prevents an adverse effect caused by a heat generation of the FET. In other words, in the former case, the structure is made such as to stop the assist in the case where the current is reduced at the first reference value so as to reach the second reference value. In the latter case, the structure is made such as to stop the assist in the case where the current becomes smaller than the current limit value. Accordingly, there is a problem that the assist frequently stops every time the voltage of the in-vehicle power supply is decreased from the reference value, particularly in the case where the output of the EPS is large. As a result, there is a problem that a reduction of an operability and a deterioration of a feeling are generated, and as a result a commodity characteristics of the EPS is significantly deteriorated.